This invention relates to nuclear reactors and has particular relationship to the replacement of old split-pin assemblies by new split-pin assemblies in the guide tubes of reactors.
Typically, a guide tube includes an upper section and a lower section which is referred to as the lower guide tube (LGT). The split-pin assemblies are mounted in the lower flange of the LGT. For each split-pin assembly, the flange includes an upper counterbore and a lower counterbore. The split pin extends into the lower counterbore with its shank extending into the upper counterbore through a hole in a partition between the counterbores. The upper part of this shank is threaded. The split pin is held by a nut which is threaded onto the threaded shank and seats on the upper surface of the partition. When the nut is threaded onto the shank of the split pin, the split pin is drawn into the lower counterbore and its flange engages the lower surface of the partition between the counterbores. For various reasons, it has become necessary to replace the split-pin assemblies which were secured in the guide tubes in the past. The assemblies to be replaced are herein referred to as old split-pin assemblies. In the old split-pin assemblies, a dowel pin passes through the nut and is welded to the nut. A split-pin assembly extends into a vertical slot similar to a "mouse hole" in the wall of the guide tube. The part of this wall which extends above and bounds this slot limits the access to the split-pin assembly.
In the replacement of an old split-pin assembly by a new assembly, it is necessary that the old assembly be removed. Both the guide tube and the old assembly are highly radioactive and they must be processed in a pool of water. The guide tube is a massive complicated body of stainless steel. Its fabrication is highly time consuming and labor intensive and it is for this reason costly; typically, the cost is about $80,000. It is accordingly indispensable that the old split-pin assemblies be removed without damaging the guide tube. It is an object of this invention to accomplish such removal of the old split-pin assembly.
The nut and split pin of an old split-pin assembly cannot be separated by untorquing the nut because the nut is welded to the dowel pin. It has been proposed that the old split-pin assembly be removed by a remotely operation robotic saw. This expedient has proven not practicable because it demands a highly complicated operation difficult to carry out, without damaging the guide tube, particularly in the limited region near the slot in the guide. It is accordingly an object of this invention to provide apparatus and a method for removing an old split-pin assembly which shall effect the removal in a relatively uncomplicated manner and shall preclude damage to the guide tube.